I was just learning about 2i/Borosov, which I guess was discovered in August but I only learned of it now. And I thought, if there were any comet we'd want to send a probe to, that would be the one, since it's not coming back and there's no telling when another extrasolar object might appear. But half a year isn't enough for a new mission, there would have to be something ready and waiting. It doesn't have to be ICBM-like, ready to go with seconds of notice. There's still time to arrange for a launch on a Delta or a Falcon or something. But the spacecraft would have to be ready to go, and a launcher would have to be ready to fit it into their schedule on short notice. Is there something like that, or plans for it? Is there a reason we couldn't have a mission ready and waiting?
Yes, or at least we will have soon. The ESA Comet Interceptor mission is designed to "lurk" at the Earth-Sun L2 for up to five years until a suitable target comes along. The target will most likely be a "new" comet on its first pass around the Sun, but could be an interstellar visitor. The linked page says:
Although much rarer, another example of a potential target is an interstellar interloper from another star system, like the famed ‘Oumuamua that flew past our Sun on a highly inclined orbit in 2017. Studying an interstellar object would offer the chance to explore how comet-like bodies form and evolve in other star systems.